1. Field of the Invention
This invention relates to a nitride-based semiconductor substrate and, in particular, to a nitride-based semiconductor substrate that has a high thermal conductivity and electron mobility while securing a sufficient electrical conductivity. Also, this invention relates to a semiconductor device fabricated using the nitride-based semiconductor substrate.
2. Description of the Related Art
Nitride-based semiconductor devices attract attention for a high-output laser diode (LD) used in a high-speed writing next-generation DVD, for a high-output light emitting diode (LED) used in a automobile headlight or a general lighting system, and for a high power conversion element. In such devices, fast dissipation of heat to be generated in high-output operation is a key issue so as to provide a high output, high efficiency and high reliability device.
Thus far, the nitride-based semiconductor devices such as LED and LD have been fabricated generally by growing epitaxial layers on a sapphire substrate by MOVPE etc.
However, since the sapphire substrate has a thermal conductivity as low as 0.42 W/cmK, heat dissipation thereof is a serious problem. In this regard, when a SiC substrate is used instead of the sapphire substrate, the heat dissipation property can be enhanced significantly since it has a thermal conductivity of about 4 W/cmK. However, light extraction efficiency thereof is lowered since the conductive SiC substrate is thickly colored into green. In addition, dislocation density thereof is still as high as the sapphire substrate.
A method of improving the heat dissipation property of LED is a flip-chip mounting that the epi-layer side of a LED chip is mounted on a stem. However, when the sapphire substrate is used, a problem occurs that the amount of light incident to the sapphire substrate as a main light extracting portion does not increase as expected since there is a big refractive index difference between the epi-layer and the sapphire substrate. As a result, light extraction efficiency thereof is not enhanced so much. Furthermore, the fabrication cost must be increased since the process is complicated.
In case of using a GaN substrate, it is possible to provide a device with a good heat dissipation property due to its high thermal conductivity without reducing the light extraction efficiency.
A high-quality GaN is reported which has a thermal conductivity as high as about 2 W/cmK (See e.g., Document 1: D. I. Florescu et al., “High spatial resolution thermal conductivity and Raman spectroscopy investigation of hydride vapor phase epitaxy grown n-GaN/sapphire (0001): Doping dependence”, Journal of Applied Physics 88(6) (2000) p 3295). This value is about five times the sapphire (0.42 W/cmK) and is a very high value close to aluminum (2.4 W/cmK).
In general, it is necessary to secure a sufficient electrical conductivity in order to reduce the operating voltage of a device or to form an ohmic contact on the surface of a GaN substrate. Therefore, doping of impurity is needed so as to have a carrier concentration of about 1.2×1018 cm−3 or more.
However, in general, the thermal conductivity of a semiconductor crystal lowers according to defect or impurity contained in the crystal. This is because phonon is dispersed by the defect or impurity or its complex. For example, Document 1 reports that the thermal conductivity of GaN is as high as 1.95 W/cmK at a carrier concentration (n) of 6.9×1016 cm−3, but it is reduced to about 0.5 W/cmK near at a carrier concentration (n) of 3.0×1018 cm−3. The latter thermal conductivity value is as low as the sapphire. Thus, the advantage of using the GaN substrate is spoiled.
In addition, since the impurity also causes the dispersion of carrier as described earlier, carrier mobility may be lowered thereby. When the carrier mobility is lowered, a high-density doping is needed to secure the same conductivity. This causes a vicious circle that the thermal conductivity lowers thereby. Thus, it is difficult to provide a GaN self-standing substrate with a high thermal conductivity and electron mobility when the carrier concentration is enhanced to secure a sufficient electrical conductivity.